Extensive changes have occurred recently in emission standards. Currently, federal and California standards for Tier I vehicles on the FTP test at 50K miles are the same (NMHC, 0.25 g/mi; CO, 3.4 g/mi; NO.sub.x, 0.4 g/mi). California has also legislated a series of increasingly stringent emission standards in order to produce a fleet of lower emission vehicles, such as low emission (LEV) and ultra-low emission (ULEV) vehicles, with even lower emission standards, particularly for hydrocarbons. The government has also enacted regulations requiring that gasoline powered vehicles in MY94 in California (e.g., CARB Mailout #91-37) and MY96 in the other 49 states (Federal Register Vol. 58, #32, Feb. 19, 1993) be able to self-diagnose a malfunction by any of several components of the emissions control system. Under more recent regulations (e.g., CARB Mailout #94-03) the on-board diagnostics (OBD II) system needs to identify deterioration that is severe enough to exceed a threshold for the entire system. For California LEV and ULEV vehicles, the threshold is exceeded if the amount of non-methane hydrocarbon (NMHC, henceforth just HC) emitted in the exhaust during an FTP test (i.e., a test in which involves about 31.2 minutes of driving on a chassis dynamometer through a given speed-time schedule that covers the equivalent of 11 miles) exceeds 1.5 times the standard under which the vehicle was certified.
Current OBD II regulations require that each component of the emissions control system be checked during each trip to ensure that it functions properly. The most difficult component to check on-board is the catalytic converter. To check the catalytic converter, the environmental regulators originally suggested the dual oxygen-sensor method. The dual oxygen-sensor method is presently used by GM and all other car manufacturers since oxygen sensors are presently the only sensors available with proven durability in exhaust. However, even oxygen sensors have durability problems as described below.
Despite the wide use of the dual oxygen-sensor method, it has a fundamental problem: the quantity it measures is the oxygen storage capability of the catalytic converter, not its HC conversion efficiency. While it is possible to distinguish between a new catalytic converter and a completely inactive catalytic converter on the basis of oxygen storage, there is not a good correlation between oxygen storage capacity and the HC conversion performance of the catalytic converter for aged parts.
As performance requirements for the catalytic converter are increased to meet more stringent emissions regulations, it becomes impossible to use the dual oxygen-sensor method to distinguish between a catalytic converter that functions improperly and a good catalytic converter that simply has lost part of its oxygen storage. The use of fuel with high sulfur content has a much more severe effect on oxygen storage than on hydrocarbon conversion efficiency, for example. Also, it has been found that under conditions of normal use, the oxygen sensor downstream of the catalytic converter can change so that it responds more slowly to variation of the oxygen concentration than it did when it was new. This has the effect of allowing a malfunctioning catalytic converter to go undetected. Although this problem should be alleviated by software/hardware modifications, it is another difficulty of the existing method.
Vehicle certification relies on the FTP test, which involves about 31.2 minutes of driving on a chassis dynamometer through a given speed-time schedule that covers the equivalent of 11 miles. Today, in an FTP test of a properly functioning vehicle, of the total quantity of HC emitted, 60% to 80% is emitted in the first 2 minutes--before the catalytic converter has warmed up enough to reach 50% conversion efficiency (light off). As the catalytic converter deteriorates, the time to light off increases.
D. R. Hamburg, "Catalyst Monitoring Using a Hydrocarbon Sensor", U.S. Pat. No. 5,177,464 discloses a system that draws exhaust gas into a remote test chamber where HC concentration is measured. Two solenoid valves are used to control whether the exhaust sampled is taken from before or after the catalytic converter. If the ratio of HC concentration before and after the catalytic converter is not below a preset threshold the catalyst is indicated as being faulty. J. A. Cook, D. R. Hamburg, E. M. Logothetis, R. E. Soltis, J. H. Visser, and M. Zanini-Fisher, "Method and Apparatus for Determining the Hydrocarbon Conversion Efficiency of a Catalytic Converter, U.S. Pat. No. 5,265,417, describes a similar system in which a catalytic differential calorimetric sensor is used to determine the concentration of HC vapor.